1. Field of the Invention
The present invention is related to the field of surface measurements and metrology. More specifically, the invention relates to optical devices for optically measuring characteristics of a surface such as roughness, waviness and/or form error.
2. Description of Related Art
The global economy and the ever increasing demands of competition has led to ever increasing quality of manufactured products. In this quest for quality, manufacturing methods and machines have been created to attain the engineering requirements specified in these products. As manufacturers seek to produce products with more desirable surface characteristics and to tightly control quality of these surfaces, technology related to the field of surface characteristics measurement and metrology have continued to develop and evolve. For example, in the metal processing industry, the measurement of surface characteristics is critical in determining the quality of ground or rolled metal products. In addition, surface characteristics measurement must be made on the mills, rollers, molds and other processing equipment which are used to manufacture metal products in order to ensure that the quality of the products meet or exceed the engineering design specifications. Of course, surface characteristics measurement is critical in many other industries in addition to the metal processing industry such as plastics, textiles, paper, composites, silicon processing and glass industries to name a few.
Various surface characteristics may be described and quantified to describe the physical attributes of any given surface. Such characteristics of particular interest include surface roughness, waviness and form error which are all currently measured and monitored in the above noted industries as well as others. These three surface characteristics all describe the irregularities which are present in all surfaces. These surface characteristics are related terms of art and are differentiated primarily by the wavelength and the amplitude of a particular irregularity such as a peak or a valley on the surface. In this regard, a reference parameter "G", has been established in order to allow this differentiation. The reference parameter G is defined as the ratio between the amplitude of the irregularity and one wavelength of the irregularity (i.e. distance between consecutive irregularities). Thus, surface roughness is generally characterized by 0.01&lt;G&lt;0.2; waviness is generally characterized in that 0.001&lt;G&lt;0.01; and form error is generally characterized in that G&lt;0.001. In absolute numeric terms, surface roughness in metals and metal manufacturing is generally considered to have a wavelength .lambda.&lt;500 .mu.m. In these industries, waviness is generally considered to have a wavelength .lambda. between 500 .mu.m and 1 cm whereas form error generally has a wavelength .lambda.&gt;1 cm. As described, it should be understood that these three surface characteristics are differentiated primarily by the size of the wavelength. Thus, the above cited measurements are general ranges only and may differ between applications and various industries.
Surface roughness has been of particular interest to various industries including the steel and machine industries. In these industries, the surface roughness is quantified by measuring an "Ra" value which is defined as the arithmetical average profile deviation of the surface irregularities with respect to a hypothetical perfect surface established by an arithmetical averaged line. Because of the importance in obtaining accurate surface roughness measurements, many devices have been developed to measure the Ra values of a surface. For instance, mechanical devices have been developed including profilometers that have a probe such as a stylus which is brought in contact with the surface of the object being measured. The stylus is then horizontally moved across the surface for a predetermined distance. During this horizontal movement, the stylus is moved in a vertical direction following the peaks and valleys of the irregularities on the surface thereby providing a profile of the surface being measured. This vertical displacement generates an electrical signal which may then be used with the known horizontal displacement to determine the surface roughness. Such profilometers are known in the art and is generally described in U.S. Pat. No. 5,778,551 to Herklotz et al.
Although these profilometers have gained substantial popularity in industry, there are several disadvantages which limit their applicability. These disadvantages include the fact that the object being measured must be physically contacted by the probe in order to obtain the roughness measurement. This contact can cause scratches and additional irregularities on the surface being measured. Other disadvantages include limitations on accuracy and repeatability since the probes have a physical dimension and will alter the surface as it is moved across the measured surface. In addition, the profilometer is not practical for use in many manufacturing settings such as in a production line because the object to be measured must be stopped and the measurement process itself takes a relatively long time. Furthermore, many manufacturing environments are subject to vibrations which can render the profilometer measurements inaccurate and useless. For these reasons, profilometers are commonly used in laboratory environments and have not been effectively implemented in manufacturing environments.
Optical devices which allow non-contact measurement of surfaces have been developed in order to avoid the above noted disadvantages of mechanical designs. These optical devices detect the image of an illuminated point such as those created by a laser beam on the surface to be measured. Two categories of such optical devices known and used in industry are light scattering systems and triangulation systems.
The light scattering systems measure a surface characteristic by measuring the amount of a light beam scattered by the surface; or conversely, by measuring the intensity of light beam not scattered by the surface. Such light scattering systems generally operate by deflecting a laser beam at a predetermined angle off the surface to be measured. This deflected laser beam is somewhat scattered by the surface irregularities thereby creating a diffused field where the light is deflected in various directions depending upon the surface irregularities. This scattering of the laser beam correspondingly decreases the intensity of the deflected specular beam. The deflected specular beam is then directed on to a photodiode which generates a signal in proportion to the intensity of the deflected specular beam. Since the intensity of the light beam initially emitted by the laser is known, the desired surface characteristic can be determined by processing the signal from the photodiode. More specifically, the signal which corresponds to the intensity of the deflected specular beam (or conversely, the reduction of the initial laser beam) may be correlated with known surface characteristics such as roughness. In other systems, the diffused fields of the laser beam may be detected by photodetectors to provide a signal corresponding to the intensity of these fields in order to determine surface characteristics. Such light scattering systems are illustrated and discussed in U.S. Pat. No. 3,771,880 to Bennett, U.S. Pat. No. 4,364,663 to Gardner et al. and U.S. Pat. No. 5,608,527 to Valliant et al. Another related light scattering system is disclosed in U.S. Pat. No. 5,661,556 to Schiff et al. which utilizes a hollow sphere to measure the total laser light scattered on a surface to determine the correlated roughness of the surface.
In contrast to the light scattering systems described above, the triangulation systems measure surface characteristics by detecting a position of diffused light on a position sensing device (PSD). More specifically, such triangulation systems operate by focusing a laser beam on a point at a predetermined work distance directly over the surface to be measured. A diffused light of the laser beam which is diffused by the surface being measured is focused on to a PSD such as a PIN diode that is sensitive to the position of the diffused light. The PIN diode produces an output signal indicative of the position of the diffused light. When the focused laser beam is over a surface irregularity such as a peak or a valley, the position of the diffused light focused on the PIN diode changes thereby changing the output signal provided by the PIN diode. The desired surface characteristic may then be measured based on the change in the output signal which is proportional to the deflection of the diffused light, which in turn, is geometrically correlated to the shape and size of the irregularity on the surface being measured. Such triangulation systems are illustrated and discussed in U.S. Pat. No. 5,617,645 to Wick et al. Other triangulation measurement systems utilize a plurality of photodiodes to obtain more accurate surface characteristic measurements. Such systems are disclosed in U.S. Pat. No. 4,973,164 to Weber et al.
Both the light scattering and the triangulation systems of the type discussed above have been found to be particularly useful in manufacturing applications because measurements do not require a probe that physically contacts the object to be measured. An added advantage is that the measurements may be made quickly (e.g. 500 measurements/minute) without interrupting or stopping the manufacturing process. However, disadvantages to these systems have also been found which limit their utility and applicability.
Initially, with respect to the light scattering systems, limitations in efficiency and precision in medium and high roughness ranges have been found. In these systems, testing has revealed that the measurement range is limited by the wavelength and the power of the laser source. As the roughness of the surface increases, the amount of light scattered also increases thereby substantially decreasing the amount of light received by the photodiode which, correspondingly, results in the decrease of the resolution and accuracy of the measurements. In order to compensate for the additional scatter, higher power lasers would have to be used. However, use of such higher power lasers may be strictly regulated or prohibited in some cases and such high power lasers are expensive making it an unviable option in most industrial applications. In addition, because of the intensity of the light beam, such high powered lasers may, in fact, alter the surface of the object being measured. Thus, studies have found that current light scattering systems using conventional, commercially available lasers are not accurate beyond an approximate Ra value of 0.4 .mu.m. In addition, such light scattering systems cannot provide accurate information regarding surface waviness and form error which have much longer wavelength .lambda. than surface roughness. In those light scattering systems that measure total diffused light with spheres, limitations have been found with respect to their effectiveness because of the difficulties associated with preventing stray light from entering the hollow sphere, containing all the light within the sphere and the restrictive physical dimensioning of the sphere and its components.
With respect to triangulation systems, it has been found that such systems are effective in measuring surface characteristics with larger wavelengths .lambda. such as waviness and form error. However, it has been found that these triangulation systems and the current PIN photodiode technology does not provide adequate resolution to effectively measure surface roughness except for very high roughness such as when Ra&gt;5 .mu.m. Hence, whereas triangulation systems are very useful in providing various surface characteristics such as waviness, form error and to a lesser extent, very high roughness, these systems have been found to be inadequate where higher resolution is required such as during roughness measurements of very smooth surfaces having a low Ra value.
Thus, because the currently known systems measure surface characteristics through either the light scattering or the triangulation techniques discussed above, they do not provide accurate measurements with respect to all of the surface characteristics including surface roughness, waviness and form error. In addition, these current systems do not provide sufficiently accurate surface roughness measurements of Ra between 0.4 .mu.m and 5 .mu.m and more specifically, fail to give accurate measurements with respect to surface roughness ranging between 0.4 .mu.m to 1.4 .mu.m which is the range of surface roughness commonly used in metal manufacturing and processing industries.
Furthermore, it has been found that many of currently available light scattering and triangulation systems require precise positioning and alignment relative to the surface being measured in order to yield accurate results. However, such precise positioning and alignment is often difficult to attain, especially in manufacturing environments where vibrations occur. Consequently, it has also been found that these systems are susceptible to misalignment errors and inaccuracies, especially in the manufacturing environments where such non-contact measurement systems would be most useful.
Therefore, there exists a need for an improved surface characteristics measurement system that can be used in a manufacturing environment and provide accurate surface characteristics measurements, especially with respect to surface roughness, waviness and form error. There also exists a need for an improved surface characteristics measurement system that can provide accurate surface roughness measurements of Ra between 0.4 .mu.m and 1.4 .mu.m. In addition, there exists a need for an improved surface characteristics measurement system which is not as susceptible to misalignment errors as the currently available systems. Furthermore, there also exists a need for a method for effectively obtaining these measurements.